Date of Award


Degree Type


Degree Name

Doctor of Philosophy (PhD)


Medical Sciences


Dr. D.H. Carr


Dr. H.I.J. Van der Spuy

Committee Member

Dr. C.A. Woodward


Extinction to Double Simultaneous Stimulation (EDSS) is a perceptual disorder which occurs with varying frequency following lateralized brain lesions, predominantly in the rifht parietal lobe. Correlated with impaired functional recovery of hemiplegic patients, the phenomenon is characterized by faulty or non-recognition of stimuli on the side of opposite to that of the damaged cerebral hemisphere where these stimuli are presented simultaneously with stimuli on the ipsilesional side. Yet the phenomenon cannot be explained by a simple sensory deficiency since a patient who manifests the syndrome correctly identifies the same stimuli on unilateral presentation. EDSS is considered a (milder) sanifestation of the 'neglect' syndrome, that is, the tendency to neglect the contralesional half of the body and space following brain damage. Although known for a full century, the psychoshysiologic mechanisms underlying the syndromes of EDSS and 'neglect' are still poorly understood and many hypotheses remain speculative. Investigation of an underlying mechanism seems essential for the development of treatment strategies aimed at ameliorating the syndrome and thereby conceivably enhancing the rehabilitation potential of this patient group. Although the unilateral neglect of stimuli may involve different sensory modalities, the present study focuses on somatosensory stimuli. The purpose of the present study was two-fold: first, to test two prevailing theories which account for the syndrome of EDSS in hemiplegic patients, and to distinguish between the respective hypotheses; second, to investigate hemispheric asymmetries in the perception of somatosensory stimuli prior to brain injury as a possible basis for the development of unilateral hemineglect after brain lesions. The postulated mechanisms underlying EDSS, stimulus properties and the hemispace in which the stimuli occur, were therefore tested in both a clinical as well as a neurologically intact population. The research questions were addressed in two parts of the study: in study A, electrical stimuli at perception threshold, systematically varied with respect to duration and the hemispace in which they were delivered, were presented to 68 subjects free of neurological disease (34 males, 34 females). The subjects; report of perceived stimuli was recorded and errors of detection were analyzed. In study B, a similar procedure was performed on 19 hemiplegic patients manifesting EDSS. The results indication that both sensory characteristics and attentional factors influence perception. The findings of study A suggest that: 1. The hemispace in which stimuli are presented play a significant role in their perception. 2. A left hemispace advantage appears to be pre-eminent in females. 3. The right hemisphere may be a better 'watchkeeper' for stimuli directed to the left hemisphere that the left hemisphere is for stimuli directed to the right hemisphere. In hemiplegic patients (study B), neither of the two tested theories could account by itself, under the experimental conditions of the present study, for the syndrome of 'extinction'. Based on the present findings, EDSS is viewed as a deficiency in neurointegrative functioning altering the physiological processing of stimuli.

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